In an oil-filled electrical apparatus such as an oil-filled transformer, coil copper is used as an energizing medium. This coil is wound with insulating paper, which provides electrical insulation to prevent an electrical short circuit in the coil between neighboring turns.
An insulating oil such as a mineral oil is used in an oil-filled transformer. It is known that a mineral oil or the like contains a small amount of a sulfur component, which reacts with the coil copper placed in the electrical insulating oil to produce electrically conductive copper sulfide on a surface of the coil insulating paper. It is known that such production of copper sulfide causes deteriorated performance of the coil insulating paper, which may lead to a short circuit in the coil between turns and thus, dielectric breakdown (Non-Patent Document 1, for example).
It is also known that the substance mainly responsible for the production of copper sulfide is dibenzyl disulfide, which is a sulfur component contained in a mineral oil (Non-Patent Document 2, for example). The mechanism of production of copper sulfide is known as follows: a complex produced by reaction of dibenzyl disulfide with the coil copper diffuses into the electrical insulating oil and adsorbs to the insulating paper, and then the complex decomposes to produce copper sulfide (Non-Patent Document 3, for example).
Methods for inhibiting production of copper sulfide by inhibiting the reaction between dibenzyl disulfide and the coil copper based on the above-described production mechanism are known, among which a method wherein an inhibitor is added to an electrical insulating oil is widely used. Examples of known inhibitors include benzotriazole compounds (Non-Patent Document 4, for example) such as 1,2,3-benzotriazole (hereinafter abbreviated to “BTA”) and Irgamet (registered trademark) 39 [N,N-bis(2-ethylhexyl)-(4 or 5)-methyl-1H-benzotriazole-1-methylamine, manufactured by BASF Japan, Inc.].
When an inhibitor for the production of copper sulfide is added to an electrical insulating oil, the inhibitor reacts with the coil copper to form a film on the surface of the coil copper. The formed film serves to block or inhibit the reaction between dibenzyl disulfide and the coil copper, thereby inhibiting the production of copper sulfide (Non-Patent Document 4, for example).
Since Irgamet 39 has a lipophilic long-chain alkyl group, it has high solubility in an electrical insulating oil as compared to BTA, and can be dissolved in an electrical insulating oil without using a special dissolving apparatus needed for the operation of adding BTA (Patent Documents 1 and 2, for example). Therefore, the addition of Irgamet 39 to an existing transformer simplifies the on-site work, leading to a shorter power-outage time on site. Thus, the needs for application of Irgamet 39 are increasing.
The addition of Irgamet 39 to a transformer, however, has a problem in that a large amount of hydrogen gas is produced (Non-Patent Documents 4 and 5, for example). Moreover, because hydrogen gas is a gas component that serves as an index of discharge at the time of an internal abnormality diagnosis of the transformer, if hydrogen gas is produced by the addition of Irgamet 39, a correct diagnosis of an internal abnormality in the transformer cannot be made.